Rubber compositions can be reinforced and improved in modulus (complex modulus) by adding thereto microfibrillated plant fibers (e.g. cellulose fibers) as filler. However, microfibrillated plant fibers have strong self-aggregation properties and poor compatibility with rubber components. For example, if microfibrillated plant fibers are added to and mixed with a rubber latex, about 20% of the microfibrillated plant fibers added still remain in the solution instead of being incorporated into the rubber component.
Also, in preparing a masterbatch by mixing a rubber latex and microfibrillated plant fibers, agglomerates of the microfibrillated plant fibers are likely to be formed in the masterbatch. Therefore, in a tire formed from such a masterbatch, the agglomerates may cause premature wear, cracks, chipping, and interlayer separation, which may even lead to air leakage and loss of handling stability.
Thus, though the addition of microfibrillated plant fibers improves modulus, it decreases the tire performance requirements of handling stability, abrasion resistance, and durability, which may result in loss of the functions and advantages of tires. Therefore, this method has room for improvement.
In general, microfibrillated plant fibers are dispersed into a solvent by a homogenizer before use. However, cylinder homogenizers have room for improvement, since they provide poor circulation of microfibrillated plant fibers, failing to uniformly defibrate the microfibrillated plant fibers. Propeller homogenizers also have room for improvement, since they have a small shearing force and therefore low defibration efficiency and also since microfibrillated plant fibers adhere to the propeller blades and the adhered fibers are thus prevented from being defibrated.
Patent Literature 1 discloses a method of chemically modifying microfibrillated plant fibers so as to improve the compatibility of the rubber component and microfibrillated plant fibers. This method, however, is not sufficient for overall improvement of the properties described above.